imap-send: return with error if curl failed

curl_append_msgs_to_imap always returned 0, whether curl failed or not.
Return a proper status so git imap-send will exit with an error code
if something wrong happened.

Signed-off-by: Nicolas Morey-Chaisemartin <nicolas@morey-chaisemartin.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>